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https://github.com/f4exb/sdrangel.git
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160 lines
5.5 KiB
C++
160 lines
5.5 KiB
C++
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///////////////////////////////////////////////////////////////////////////////////
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// Copyright (C) 2016 Edouard Griffiths, F4EXB //
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// //
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// This program is free software; you can redistribute it and/or modify //
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// it under the terms of the GNU General Public License as published by //
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// the Free Software Foundation as version 3 of the License, or //
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// //
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// This program is distributed in the hope that it will be useful, //
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// but WITHOUT ANY WARRANTY; without even the implied warranty of //
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the //
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// GNU General Public License V3 for more details. //
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// //
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// You should have received a copy of the GNU General Public License //
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// along with this program. If not, see <http://www.gnu.org/licenses/>. //
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///////////////////////////////////////////////////////////////////////////////////
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#include "util/CRC64.h"
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/**
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* poly is: x^64 + x^62 + x^57 + x^55 + x^54 + x^53 + x^52 + x^47 + x^46 + x^45 + x^40 + x^39 +
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* x^38 + x^37 + x^35 + x^33 + x^32 + x^31 + x^29 + x^27 + x^24 + x^23 + x^22 + x^21 +
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* x^19 + x^17 + x^13 + x^12 + x^10 + x^9 + x^7 + x^4 + x^1 + 1
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*
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* represented here with lsb = highest degree term
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*
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* 1100100101101100010101111001010111010111100001110000111101000010_
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* || | | || || | | |||| | | ||| | |||| ||| |||| | | |
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* || | | || || | | |||| | | ||| | |||| ||| |||| | | +- x^64 (implied)
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* || | | || || | | |||| | | ||| | |||| ||| |||| | |
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* || | | || || | | |||| | | ||| | |||| ||| |||| | +--- x^62
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* || | | || || | | |||| | | ||| | |||| ||| |||| +-------- x^57
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* .......................................................................
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* ||
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* |+---------------------------------------------------------------- x^1
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* +----------------------------------------------------------------- x^0 (1)
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*/
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const uint64_t CRC64::m_poly = 0xC96C5795D7870F42ull;
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CRC64::CRC64()
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{
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build_crc_table();
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}
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CRC64::~CRC64()
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{}
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/**
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* input is dividend: as 0000000000000000000000000000000000000000000000000000000000000000<8-bit byte>
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* where the lsb of the 8-bit byte is the coefficient of the highest degree term (x^71) of the dividend
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* so division is really for input byte * x^64
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*
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* you may wonder how 72 bits will fit in 64-bit data type... well as the shift-right occurs, 0's are supplied
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* on the left (most significant) side ... when the 8 shifts are done, the right side (where the input
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* byte was placed) is discarded
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*
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* when done, table[XX] (where XX is a byte) is equal to the CRC of 00 00 00 00 00 00 00 00 XX
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*/
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void CRC64::build_crc_table()
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{
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for(int i = 0; i < 256; ++i)
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{
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uint64_t crc = i;
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for(unsigned int j = 0; j < 8; ++j)
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{
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if(crc & 1) // is current coefficient set?
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{
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crc >>= 1; // yes, then assume it gets zero'd (by implied x^64 coefficient of dividend)
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crc ^= m_poly; // and add rest of the divisor
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}
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else // no? then move to next coefficient
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{
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crc >>= 1;
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}
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}
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m_crcTable[i] = crc;
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}
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}
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/**
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* will give an example CRC calculation for input array {0xDE, 0xAD}
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*
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* each byte represents a group of 8 coefficients for 8 dividend terms
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*
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* the actual polynomial dividend is:
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*
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* = DE AD 00 00 00 00 00 00 00 00 (hex)
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* = 11011110 10101101 0000000000000000000...0 (binary)
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* || |||| | | || |
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* || |||| | | || +------------------------ x^71
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* || |||| | | |+-------------------------- x^69
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* || |||| | | +--------------------------- x^68
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* || |||| | +----------------------------- x^66
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* || |||| +------------------------------- x^64
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* || ||||
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* || |||+---------------------------------- x^78
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* || ||+----------------------------------- x^77
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* || |+------------------------------------ x^76
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* || +------------------------------------- x^75
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* |+--------------------------------------- x^73
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* +---------------------------------------- x^72
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*
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*
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* the basic idea behind how the table lookup results can be used with one
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* another is that:
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*
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* Mod(A * x^n, P(x)) = Mod(x^n * Mod(A, P(X)), P(X))
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*
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* in other words, an input data shifted towards the higher degree terms
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* changes the pre-computed crc of the input data by shifting it also
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* the same amount towards higher degree terms (mod the polynomial)
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*
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* here is an example:
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*
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* 1) input:
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*
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* 00 00 00 00 00 00 00 00 AD DE
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*
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* 2) index crc table for byte DE (really for dividend 00 00 00 00 00 00 00 00 DE)
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*
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* we get A8B4AFBDC5A6ACA4
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*
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* 3) apply that to the input stream:
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*
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* 00 00 00 00 00 00 00 00 AD DE
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* A8 B4 AF BD C5 A6 AC A4
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* -----------------------------
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* 00 A8 B4 AF BD C5 A6 AC 09
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*
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* 4) index crc table for byte 09 (really for dividend 00 00 00 00 00 00 00 00 09)
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*
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* we get 448FCBB7FCB9E309
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*
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* 5) apply that to the input stream
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*
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* 00 A8 B4 AF BD C5 A6 AC 09
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* 44 8F CB B7 FC B9 E3 09
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* --------------------------
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* 44 27 7F 18 41 7C 45 A5
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*
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*/
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uint64_t CRC64::calculate_crc(uint8_t *stream, int length)
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{
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uint64_t crc = 0;
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for (int i = 0 ; i < length; ++i)
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{
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uint8_t index = stream[i] ^ crc;
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uint64_t lookup = m_crcTable[index];
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crc >>= 8;
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crc ^= lookup;
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}
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return crc;
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}
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